1. Field of the Invention
This invention relates to new drilling fluids based on ester oils and to invert drilling muds based thereon which combine high ecological compatibility with good stability and performance properties.
2. Statement of Related Art
It is known that liquid drilling fluids for sinking bores in rock and bringing up the rock cuttings are slightly thickened, water-based, or oil-based fluid systems. Oil-based systems are being increasingly used in practice particularly in offshore drilling or in the penetration of water-sensitive layers.
Oil-based drilling fluids are generally used in the form of so-called invert emulsion muds which consist of a three-phase system, namely: oil, water and finely divided solids. Such emulsions are of the w/o emulsion type, i.e. the aqueous phase is present in the continuous oil phase in heterogeneous fine dispersion. There are a whole range of additives, including in particular emulsifiers and emulsifier systems, weighting agents, fluid loss additives, alkali reserves, viscosity regulators and the like, for stabilizing the system as a whole and for establishing the desired performance properties. Full particulars can be found, for example, in the Article by P. A Boyd et al entitled "New Base oil Used in Low-Toxicity oil Muds" in Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R. B. Bennet entitled "New Drilling Fluid Technology--Mineral Oil Mud" in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein.
Oil-based drilling fluids were originally made from diesel oil fractions containing aromatic constituents. For the purposes of detoxification and reducing the ecological problems thus created, it was then proposed to use hydrocarbon fractions substantially free from aromatic compounds --now also known as "nonpolluting oils"--as the continuous oil phase, cf. the literature cited above. Although certain advances were achieved in this way through elimination of the aromatic compounds, a further reduction in the environmental problems caused by drilling fluids of the above type seems to be urgently required. This applies in particular to the sinking of offshore wells for the development of oil and gas sources because the marine ecosystem is particularly sensitive to the introduction of toxic and non-readily degradable substances.
The relevant technology has for some time recognized the significance of ester-based oil phases for solving these problems. Thus, U.S. Pat. Nos. 4,374,737 and 4,481,121 describe oil-based drilling fluids in which nonpolluting oils are said to be used. Non-aromatic mineral oil fractions and vegetable oils of the peanut oil, soybean oil, linseed oil, corn oil and rice oil type, and even oils of animal origin, such as whale oil, are mentioned alongside one another as nonpolluting oils of equivalent rank. The ester oils of vegetable and animal origin mentioned here are all triglycerides of natural fatty acids which are known to be environmentally safe and which, ecologically, are distinctly superior to hydrocarbon fractions, even where they have been de-aromaticized.
Interestingly, however, not one of the Examples in the U.S. patents cited above mentions the use of such natural ester oils in invert emulsion drilling muds. Mineral oil fractions are used throughout as the continuous oil phase.
In its general descriptive part, U.S. Pat. No. 4,481,121 mentions not only triglycerides, but also a commercial product "Arizona 208" of the Arizona Chemical Company, Wayne, N.J., which is a purified isooctyl-monoalcohol ester of high-purity tall oil fatty acids. An ester of a monofunctional alcohol and monofunctional carboxylic acids, mentioned for the first time here, is described as equivalent to triglycerides of natural origin and/or dearomaticized hydrocarbon fractions.
The cited U.S. patent does not contain any reproducible Examples relating to the use of such an ester of monofunctional components.